The present invention relates to a target acquisition optoelectronic system with a very wide field, i.e., a system for the detection and the angular localization of a target within a very wide field of view, of about a half-space.
Such a system can be mounted in an aircraft. It allows then to detect enemy threats of the airplane or missile type.
Most of such optoelectronic surveillance systems utilize techniques for detecting the infrared radiation emitted by the targets. An infrared detector is placed in the focus of an optics. The role of the detector is to convert into an electrical signal the electromagnetic radiation emitted by the target and focused by the optics. By way of example, such systems are sensitive to optical wavelength ranges between 2 and 5 micrometers, or alternatively between 8 and 13 micrometers. The electrical signal produced at the detector is amplified. Electronic means for processing this signal allow to sense in it a sudden increase, corresponding to the passage of the image of a target at the infrared detector.
Infrared detectors generally have a sensitive area where the infrared flux must be focused to be converted into an electrical signal. This area has a small geometrical size. Typically, the size of the sensitive area is smaller than 1 mm.sup.2, and often decrease to 50 .mu.m.times.50.mu.m. As a result, the field in which the target must be located to be detected is very small in the case of the use of a single detector placed in the focus of a lens. The field angle is equal to the quotient of the dimension of the sensitive area and the focal length of the lens, and is typically less than one angular degree. As this value of field angle does not allow to detect a target under favorable conditions, in general several sensitive areas are juxtaposed along a longitudinal axis to form a linear array of elemental infrared detectors. Typically, a few dozens of detectors are thus grouped. Placed in the focal plane of the lens, such a linear array of detectors covers a field having an angle close to a dozen of degrees along the longitudinal direction of the array, and a fraction of a degree along a direction perpendicular to the array.
Scanning an area in space is then performed by shifting the elemental field covered by the array, this field being displaced in the direction perpendicular to the longitudinal axis of the array.
Referring to FIG. 1, there is shown schematically a known acquisition system, often used, including a sensor assembly comprising a lens and a linear array of infrared detectors, and mounted so that it can rotate about the vertical, longitudinal axis of the array. Each of the elemental detectors is connected to processing means. The target is localized angularly in an angular "strip" thus scanned as soon as the processing means have detected an increase in the electrical signal provided by one of the detectors of the array thanks, for one, to the knowledge of the number of the detector having sensed the luminous flux and, for another, to the measurement of the angular direction aimed at by the system at the time of detection.
This type of optoelectronic infrared search system has the disadvantage of being highly limited in overall scanning field.
Other concepts have been tried. They use scanning opto-mechanical scanning devices of the oscillating mirror, rotary mirror polygone, or similar types used by pairs so as to scan several superposed strips. Even with these devices, scanning a full half-space, corresponding to 2 .pi. steradians, is not possible.